Profile a blender file to optimize render speed and quality
The Cycles Roadmap mentions the following goal:


  Statistics: for power users to investigate why rendering performance is slow, why memory usage is high, which objects or materials to optimize, etc. This would be a log generated that could be shown in Blender or as a HTML report, and possibly also debugging AOVs.


However, this goal currently (December 2016) is not scheduled to happen soon, it is in the 


  things we think are very important, but that no one at the meeting planned to work on within a specific time frame. 


category.